1. Field of the Invention
The present invention relates to a method and system for bidirectional data transmission using a single transmission line, and more specifically, to a method and system for bidirectional data transmission by a time-compression multiplexing technique.
2. Description of the Related Art
As video-on-demand (VOD) services become a topic of interest today, such technologies are under study that distribute video information to subscriber premises using existing data transmission media. Transmission lines for this purpose include a single optical fiber, coaxial cable, or metallic two-wire cable, which will carry data from an information service provider to subscribers. (Hereafter, the data direction from the provider to the subscribers will be called "downstream direction," and the other direction will be called "upstream direction.")
There are several methods of full-duplex subscriber loop communications using a single metallic two-wire cable that are presently available. A time-compression method (also known as a "ping-pong" method) and an echo cancellation method are well-known techniques that enable such communications.
FIG. 14 is a block diagram showing a basic structure of the "ping-pong" transmission system for the Integrated Services Digital Network (ISDN). A transmission line 2 is terminated at both ends by a master unit 50 and a slave unit 60. The master unit 50 is communication equipment for providing information such as video data, and the slave unit 60 is communication equipment located at each subscriber's premise for receiving the information.
For data transmission purposes, the master unit 50 employs a data rate converter 51 for stepping up the transmission data signaling rate and a transmitter circuit 54 for amplifying the transmission data signal with increased data rate. For data reception purposes, the master unit 50 employs a receiver circuit 55 for receiving and amplifying an incoming data signal from the transmission line 2 and a data rate converter 53 for stepping down the data signaling rate of the received data signal. Based on its internal clock, a transmission/reception (TX/RX) controller 52 provides a timing control for switching between transmission and reception modes. Under the control of this TX/RX controller 52, a switch 56 alternates the signal transmission and reception.
The slave unit 60, on the other hand, employs a data rate converter 61 for stepping up the transmission data signaling rate and a transmitter circuit 64 for amplifying the accelerated transmission data signal for data transmission. For data reception purposes, the slave unit 60 employs a receiver circuit 65 for receiving and amplifying an incoming data signal from the transmission line 2 and a data rate converter 63 for stepping down the data rate of the received data signal. A TX/RX controller 62 provides a timing control for switching the signal transmission and reception in the slave unit 60 in synchronization with the TX/RX switching in the master unit 50. Under the control of this TX/RX controller 62, a switch 66 alternates the signal transmission and reception.
With the above-described system, the master unit 50 can deliver the video information and other data.
FIG. 15 is a diagram showing transmission signals in a ping-pong transmission system, in which the downstream transmission signal is sent from the master unit 50 (FIG. 14) as an information service provider to the slave unit 60 as an end-user and the upstream transmission signal is sent in the other direction. FIG. 15 shows some burst data transmission cycles, each of which consists of one upstream signal transmission period and one downstream signal transmission period.
In this data transmission system, the downstream and upstream signal transmission periods have the same length. This means that the signals are transferred on the downstream and upstream paths at the same data rate.
One of the prime objects of VOD systems is delivery of video information to subscribers. Therefore, from the viewpoint of efficient use of a limited capacity of the data transmission system, the downstream channel capacity should preferably be larger than the upstream channel capacity. However, the above-described ping-pong method is not always suitable for the VOD services, since the downstream and upstream channel capacities are designed to be symmetric for more general applications such as ISDN.
For adaptation to a wide variety of services, asymmetric data transmission systems are demanded, because they can assign a higher capacity to the downstream channel than to the upstream channel. For example, Asymmetric Digital Subscriber Lines (ADSL) architecture is under development for future use in North America. With a frequency-division multiplexing technique, this ADSL will provide a higher downstream channel capacity over existing subscriber loops.
In the VOD systems that apply such a frequency-division multiplexing technique to data transfer over a single transmission line, the downstream channel uses much wider bandwidth than the upstream channel. The wider bandwidth makes the downstream channel capacity higher than that of the upstream channel, thus enabling the VOD systems to achieve its prime object (i.e., delivery of video-based information to subscribers).
On the other hand, however, large scale hardware is necessary to realize the asymmetric bidirectional data transfer by using conventional frequency-division multiplexing techniques including ADSL, and this will lead to a cost problem. As a matter of fact, some of the optical fiber-based or coaxial cable-based VOD systems use a separate transmission line to send upstream control information from the subscribers. In this case, each subscriber connection requires two communication channels for the upstream and downstream transmissions. Such systems require rather large capital investment and are not economical at all.
As described above, in any conventional methods to realize an asymmetric bidirectional data transmission, large-scale equipment is inevitable. To solve this problem, it is important to develop a system that allows an asymmetric bidirectional data transmission in more economical way, fully using the existing transmission line. Moreover, the development of bidirectional data transfer techniques using a single transmission line will also contribute to finding an economical way to use an optical fiber cable for sending information to subscribers.